Histamine antagonism

DRUG ANTAGONISM
HISTAMINE
AND
IT S ANTAGONISTS
Histamine is a naturally occurring amine formed from the amino acid histidine and found in most body tissue in an inactive bound form, often in mast cells.
The active free histamine is released from cells in response to stimuli such as:
physical trauma
antigen-antibody reactions
Chemicals like
proteolytic enzyme
snake venom
insect bite
detergents
drugs like : tubocurarine, morphine, vancomycin.

The actions of histamine which are clinically important are those on
smooth muscle
blood vessels
IV histamine as little as 0.1 mg cause:
systemic vasodilatation
rapid fall in blood pressure
acceleration in HR
elevation in C.S.F pressure
flushing of the face
headache
gastric acid secretion is stimulated.
These effects last only a few minutes.
Injection of 10 mg of histamine intradermally produce the triple response of Lewis which include:
1- Flush: which is a localized erythema due to vasodilatation.
2-Wheal: localized edema due to the increase in capillary permeability
3-Flare: is arteriolar dilatation cause erythema, due to an axon reflex.

skin
secretary glands
Histamine has no effects when taken by mouth because it is metabolized rapidly by histaminase in G.I.T